This invention relates to new and useful improvements in pneumatic servo valves used with automatic web guiding systems.
Automatic web guiding systems are used to control the lateral position of moving webs of material, such as paper, plastics, textiles and other flexible materials being processed in continuous or roll form. A signal is generated by the position of the web edge relative to a sensor, and the signal output either directly controls an actuator connected to positioning means or indirectly controls an actuator through a servo valve which is connected to a web positioner. Positioners maintain the edge of the web at a desired location.
Various types of mechanical servo valves have been incorporated into automatic web guiding systems to control the operation of actuator means. As shown in U.S. Pat. No. 5,664,738 utilizes a pneumatic system having a feeler type sensor that rides on the web edge and incorporates a rotary valve that opens and closes over two matching orifices, limitations of this valve are due to the "feeler" being restricted to use on light plastic films and flimsy webs because of its mass. U.S. Pat. No. 4,609,012 is another pneumatic system having a moving suspension valve with two matching recesses that are spaced above a stationary block providing four way portage for proportional pneumatic flow for controlling actuator means. Due to the design of this valve it is limited in velocity and force, and uses excessive air. as well as being costly to manufacture and assemble. Both pneumatic systems are used in conjunction with double acting actuators.
The idea of spool type servo valves is not new or unique to automatic web guiding. Systems have incorporated four way hydraulic valves, providing proportional flow across the valve to a double acting actuator means. This system usually incorporates a low pressure, non contact pneumatic sensor which is highly desirable for sensing thin, delicate webs, that would resist direct contact from "feeler" type sensor.
Most all-pneumatic systems have not used spool type servo valves due to air loss across valve and lack of ability to achieve proportional stable control and web guiding accuracy. Air tight sealing would be needed to achieve control force at the actuator, which would create excessive friction by the sliding spool assembly, resulting in sticking or inability to generate adequate signal force, system stability and accuracy would be jeopardized, as well as excessive air usage.